1. Field of the Invention
The present invention is generally in the field of night vision devices (NVD's) of the light-amplification type. Such NVD's employ an image intensifier tube (I.sup.2 T) to receive photons of light from a scene. This scene may be illuminated by full day light; or alternatively, the scene may be illuminated with light which is either of such a low level, or of such a long wavelength (i.e., infrared light), or both, that the scene is only dimly visible or is effectively invisible to the natural human vision. The I.sup.2 T of such an NVD responsively provides a visible image replicating the scene. The present I.sup.2 T has an optimized microchannel plate (MCP), which provides a combination of high resolution, efficiently achieved electron gain level, and lowed requirement for operating voltage. This combination was previously unobtainable in the art.
2. Related Technology
Even on a night which is too dark for natural human vision, invisible infrared light is richly provided in the near-infrared portion of the spectrum by the stars of the night sky. Human vision cannot utilize this infrared light from the stars because the infrared portion of the spectrum is invisible for humans. Under such conditions, a night vision device (NVD) of the light amplification type can provide a visible image replicating a night-time scene. Such NVD's generally include an objective lens which focuses light from the night-time scene through the transparent light-receiving face of an image intensifier tube (I.sup.2 T). At its opposite image-output face, the I.sup.2 T provides a visible image, generally in yellow-green phosphorescent light. This image is then presented via an eyepiece lens to a user of the device.
A contemporary NVD will generally use an I.sup.2 T with a photocathode (PC) behind the light-receiving face of the tube. The PC is responsive to photons of infrared light to liberate photoelectrons. Because an image of a night-time scene is focused on the PC, photoelectrons are liberated from the PC in a pattern which replicates the scene. These photoelectrons are moved by a prevailing electrostatic field to a microchannel plate (MCP) having a great multitude of microchannels, each of which is effectively a dynode. That is, these microchannels have an interior surface substantially defined by a material providing a high average emissivity of secondary electrons. In other words, each time an electron (whether a photoelectron or an electron previously emitted by the microchannel plate) collides with this material at the interior surface of the microchannels, more than one electron (i.e., secondary-emission electrons) leaves the site of the collision. This process of secondary electron emissions is not an absolute in each case, but is a statistical process having an average emissivity of greater than unity.
As a consequence, the photoelectrons entering the microchannels cause a cascade of secondary-emission electrons (which provide substantially a geometric multiplication in response to the photoelectrons) moving along the microchannels, from one face to the other of the MCP. The result is a spatial output pattern of electrons from the MCP (which replicates the input pattern; but at a very considerably higher electron density) issuing from the microchannel plate.
This pattern of electrons is moved from the microchannel plate to a phosphorescent screen electrode by another electrostatic field. When the electron shower from the photocathode impacts on and is absorbed by the phosphorescent screen electrode, a visible image is produced. This visible image is passed out of the tube through a transparent image-output window for viewing.
The necessary electrostatic fields for operation of an I.sup.2 T are provided by an electronic power supply. Usually a battery provides the electrical power to operate this electronic power supply so that many of the conventional NVD's are portable. However, other sources of electrical power may be utilized to operate NVD's.
A goal that has long existed in the art of night vision devices is to improve the resolution provided by devices using I.sup.2 T's. The resolution of NVD's using an I.sup.2 T is essentially determined by the resolution of the tube itself, and this tube resolution is strongly influenced by the size and spacing dimension of the microchannels in the MCP of the I.sup.2 T. As a result, the art has sought over many years to progressively make both the size and the spacing dimension of the microchannels in MCP's of I.sup.2 T's smaller and smaller. However, this effort has met with only limited success prior to this invention.
Several examples of contemporary MCP's (these being of 18 mm nominal diameter), their thickness, channel size, channel spacing dimension, open area ratio (OAR), and length-to-diameter (L/D) ratio may be seen in the following table.
TABLE 1 MCP channel channel L/D Thickness size spacing OAR ratio Example 1 16.3 mil 8.0 .mu. 10.9 .mu. 48.9% 51.8:1 Example 2 12.3 mil 8.1 .mu. 9.7 .mu. 63.3% 38.6:1 Example 3 12.3 mil 8.0 .mu. 8.9 .mu. 73.3% 39.1:1 Example 4 12.0 mil 4.6 .mu. 5.8 .mu. 57.1% 66.3:1 Example 5 12.6 mil 4.7 .mu. 5.93 .mu. 57.0% 68.4:1 Example 6 11.3 mil 4.86 .mu. 5.96 .mu. 60.3% 59.1:1 Example 7 12.3 mil 4.9 .mu. 5.9 .mu. 62.6% 63.8:1
As can be readily seen from the above actual examples of the conventional art, conventional MCP's with relatively low resolution by current standards (i.e., with microchannels of about 8.mu.size) may achieve a L/D (i.e., length-to-diameter) ratio for the microchannels of about 40 or a little less, with an OAR (open area ratio--expresses as a percentage) for the MCP of from the low 60's to the low 70's. However, such MCP's do not provide the image resolution desired for future NVD's. On the other hand, conventional MCP's with improved resolution (i.e., with microchannels of about 5.mu.size) do not have quite as good of OAR, and have excessive L/D ratios (i.e., of about 60 or higher). The L/D ratio of a MCP is also an indication of the thickness of the MCP, since the microchannels extend from one face of the MCP to the other. It is also an indication, generally, of the required operating voltage for the MCP, since this voltage increases with increased thickness of the MCP.
Those ordinarily skilled in the art will understand that reductions in both microchannel size and microchannel spacing dimensions also have desirable and beneficial effects on other image quality factors. Two of these image quality factors that are favorably affected by reductions in microchannel size and spacing dimensions are known as fixed pattern noise and dark-multi-boundary noise. These image quality factors have to do with the "graininess" and mosaic effect visible in the image produced by an image intensifier tube.
A limiting problem with conventional I.sup.2 T's is the desirable corresponding decrease in thickness of the MCP at the same time that the microchannels are made smaller. That is, the microchannels of a conventional MCP have a length (L) to diameter (D) ratio that conventionally falls within a selected range in order for the MCP to provide a desired level of electron gain. Because the differential voltage (i.e., the operating voltage) of an MCP depends in large part on its thickness, making MCP's thinner would have a beneficial effect because their required operating voltages would be lower. However, conventional MCP's cannot be made as thin as desired because if they are so thin, then they will not survive the conventional manufacturing processes, or will not have sufficient strength to survive common effects, such as: physical shocks, vibrations, handling, and thermal cycling, all of which they are necessarily subjected to in manufacturing and in use.
Accordingly, even those conventional MCP's, which presently have a microchannel size and spacing dimension that is larger than desired for better resolution, must still be made thicker than desired, and they also require a correspondingly higher operating voltage than is optimum. It follows that conventional NVD's using such MCP's must therefore have power supplies which supply operating voltages to the I.sup.2 T's of these devices which are both higher than desired, and which are more expensive than is desired. Image intensifier tubes which are forced to operate at excessively high voltage levels across the MCP also suffer from reductions in manufacturing yield due to the possibility of internal arcing in the tubes, and with a reduction in image resolution because tube components have to be spaced further away from one another in order to minimize the possibility for this internal arcing. It will be understood by those ordinarily skilled in the pertinent arts that spacing of the MCP from other internal structures of an I.sup.2 T (such as the PC and screen electrode) has a strong effect on image resolution, with reduced spacing being desired for improved resolution.
An additional understanding of this problem of excessive thickness and excessively high operating voltage for the MCP's of conventional image intensifier tubes may be achieved by consideration of the fact that the desired level of electron gain within an MCP (i.e., the ratio of electrons issuing from the MCP to photoelectrons received from the PC) is a function of the applied voltage level to the MCP. A conventional indication of the gain ratio for conventional MCP's is provided by a theoretical standard used for some years in the industry. This standard is generally referred to as the "Universal Gain Curve" (UGC), a commonly accepted example of which is set out herein as FIG. 4. This UGC suggests that as the size of the microchannels in a MCP is decreased, the thickness of the MCP should shrink correspondingly so that the MCP becomes thinner in order to operate with a particular level of electron gain. This relationship of the thickness of the MCP to the diameter of the microchannels is referred to as an L/D ratio (the same L/D ratio used in Table 1), where "L" is the length of the microchannels (i.e., substantially equal to the thickness of the MCP--allowing for some difference because of an intentional angulation of the microchannels relative to a perpendicular to the faces of the MCP), and "D" is the diameter of the microchannels. The theoretical operating voltage for an MCP depends according to the UGC upon the L/D ratio of the microchannels, and the resulting thickness of the MCP. But, until now thin, high-resolution, low-voltage MCP's have been merely theoretical because conventional technology cannot provide MCP's which have high resolution and meet the theoretical thickness and voltage criteria of the UGC.
The differential voltage required across a MCP in order to achieve the best theoretical electron gain is a function of the L/D ratio of the MCP. However, making a conventional MCP as thin as the UGC suggests for MCP's with small microchannels has never before been successful for several reasons. Principal among these reasons is a distortion (i.e., warping and curling) of the conventional MCP's under essential processing conditions (i.e., such as electron beam scrubbing, and hydrogen activation necessary, respectively, for the MCP to be sufficiently clean of indigenous gas molecules, and to be responsive to photoelectrons to release secondary-emission electrons).
Considering the materials from which conventional MCP's are made, those ordinarily skilled will know that a glass commonly used to make conventional MCP's is known as 8161, and is available commercially from Corning Glass Works. This 8161 glass is a high-lead glass that is also rather high in potassium oxide and sodium oxide. Conventional MCP's, when attempts are made to construct these MCP's according to the theoretical indications of the UGC, distort excessively during processing; and generally will not withstand the shock, vibration, handling, and thermal cycling requirements for a practical MCP. In some cases, attempts to make such a conventional MCP with a thickness approaching that suggested by the UGC have resulted in many of the MCP's simply falling apart (i.e., crumbling) merely as a result of the rigors of manufacturing. Even if they survived the rigors of the manufacturing process, such conventional MCP's would never be considered for use in an image intensifier tube or in a NVD because they are impossibly frail.
Moreover, and in contrast to a MCP embodying the present invention, conventional I.sup.2 T's have not been able to utilize a MCP with a microchannel size and channel spacing as small at that achieved by the present invention, in combination with a MCP L/D ratio and plate thickness that is as thin as achieved by the present invention. Consequently, conventional MCP's have necessarily been made thicker than desired in order to survive the necessary processing steps, and have consequently required a higher than desired operating voltage to be applied to the MCP's. A MCP according to the present invention, in contrast, allows a considerably lower operating voltage.
Problems encountered with the conventional cladding glasses for MCP's (i.e., 8161 glass, and other conventional glasses as well) include, for example, excessive warping of MCP's during processing, an inability for the MCP work pieces to survive the necessary processing (i.e., sometimes even self-destruction because of the rigors of the manufacturing process itself), and an inability to survive necessary shock, vibration, handling, and thermal cycling requirements essential for manufacturing and use of I.sup.2 T's and NVD's.
U.S. Pat. No. 3,720,535, issued Mar. 13, 1973; U.S. Pat. No. 3,742,224, issued Jun. 26, 1973; and U.S. Pat. No. 3,777,201, issued Dec. 4, 1973 provide examples of microchannel plates or image intensifier tubes having a microchannel plate.
An additional understanding of some of the limitations of conventional MCP's may be obtained now from a consideration of FIG. 5 (PRIOR ART). Viewing now FIG. 5, a conventional microchannel plate (MCP) 10 is depicted. Microchannel plate 10 includes a circumferential solid-glass rim portion 10a, and within this rim portion in an active area of the MCP, defines a plurality of angulated microchannels 12, which each open on the electron-receiving face 14 and on the opposite electron-discharge face 16 of the MCP 10. Microchannels 12 are separated by passage walls 18. The passage walls 18 each have a respective facial area when viewed in a direction perpendicular to the plane of FIG. 5, so that in facial view of the MCP 10 a web area (indicated by arrowed reference numeral 20) is cooperatively defined by these walls. Usually, the web area 20 of a MCP will be somewhat more or somewhat less than about 50% of the active area of the MCP 10 (recalling Table 1 above). It is to be understood that the MCP 10 has an inactive circumferential rim (not seen in the drawing Figures, but indicated by the arrowed numeral 10a) that provides for mounting and electrical connection to the MCP, but which rim portion does not include microchannels and is not active in the sense of providing secondary-emission electrons. At least a portion of the surface of the passage walls 18 bounding the channels 12 is defined by a material 18a, which is an emitter of secondary electrons.
Still viewing FIG. 5, it is seen that each face 14 and 16 of the MCP 10 carries a conductive electrode layer 22 and 24, respectively. These conductive electrode layers may be metallic, or may be formed of other conductive material so as to distribute an electrostatic charge over the respective faces of the microchannel plate 10. Importantly, the electrode layers 22 and 24 are utilized to apply a differential voltage across the MCP 10, as is indicated on FIG. 5 by the symbols V+, and V-, although it will be understood that these voltage indications are merely relative, and that neither voltage may actually be positive relative to ground.
For purposes of illustration and comparison, the microchannels 12 may have a diameter of approximately 5.mu.-inch, on a spacing dimension of approximately 6.mu.-inch, with a L/D ratio of about 60, and a MCP thickness of about 12 mils. This thickness is about 1.5 times the thickness for this MCP that would be indicated as optimum by the UGC. As a result, the MCP 10 must be operated with a differential voltage (i.e., V- to V+) of about 1100 to 1200 volts. This is an undesirably high operating voltage for the MCP 10. However, were the MCP manufactured according to the conventional teachings, and were it made with a thinner L/D ratio, then it is well understood in the art that this MCP would suffer from one or more of the deficiencies explained above. That is, the MCP 10 would undoubtedly suffer from warping and distortion during manufacturing, or would suffer from an inability to withstand shock, vibration, handling, and thermal cycling in use. With respect to the warping problem, it is generally known that with conventional glasses of the type that are traditionally used to make MCP's, if the L/D ratio is too thin then the active area of the MCP will shrink during manufacturing, drawing the rim portion 10a into a wavy or warped-disk shape. Thus, the conventional high-resolution MCP 10 (recalling examples 4-7 above) has heretofore always been made thicker than the desired L/D ratio so that the rim portion 10a will have sufficient strength to resist this warpage and provide a satisfactorily flat MCP.
Understandably, if in the interest of improving image resolution even more, a conventional type of MCP (like MCP 10) were made with even smaller microchannels than is indicated in the discussion above of FIG. 5, then the thickness "T" of this modified MCP would still need to be about the same (i.e., about 12 mils) in order for the rim portion of the modified MCP to be sufficiently strong to prevent warping of the MCP during manufacturing. Thus, it would be seen that the L/D ratio of this hypothetical "improved" MCP would be even higher (i.e., even higher than about 60), and the deviation of the "improved" MCP from the theoretical optimum operating voltage indicated by the UGC would be even greater. Thus, the quandary facing designers of MCP's at present is clearly presented. Further, the necessary operating voltage for the "improved" MCP would still be about the same as that required by MCP 10 because of the similar thickness for the "improved" MCP. A device using such a MCP probably would not benefit much, if at all, from the smaller size of the microchannels of such a MCP because of the internal component spacing necessitated by the required high MCP operating voltage.